Variation in depth of water uptake for Pinus sylvestris var. mongolica along a precipitation gradient in sandy regions

Abstract

Mongolian pine (Pinus sylvestris var. mongolica) has been widely used in vegetation restoration and for windbreaks in sandy regions of northern China where water is the principal factor limiting tree survival and growth. An understanding of water use in Mongolian pine plantations is critical for effective vegetation restoration. To determine water sources in a Mongolian pine plantation, we investigated the stable isotopic ratios of δ18O and δD in precipitation, groundwater, in soil water in different soil layers, and in tree xylem at different tree ages along a precipitation gradient and in different micro-landforms. The results indicated that the main water sources for Mongolian pine were precipitation-derived shallow soil water in semi-humid regions and that the contribution proportion decreased with an increase in tree age. With reduced precipitation in semi-arid and arid regions, contribution of deep soil water to Mongolian pine water use gradually increased, and differences in soil water contribution among trees due to tree age also significantly increased. Water use patterns in trees planted in different micro-landforms (e.g. sand dune crest or inter-dune lowland) became more distinct as precipitation gradually decreased across humid to semiarid and arid regions. Moreover, Mongolian pine, including 15-, 20-, 25-, and 45-year-old trees, similar height and DBH without groundwater in semi-arid climate (in Ejin Horo banner), while trees planted in extremely arid climate (Linze) relied on irrigation for survival despite abundant and available groundwater at that study site. These results indicated that groundwater may be beyond reach to be a main water source for Mongolian pine trees. Soil water conditions determined the survival and sustainable growth of planted trees, especially at depths less than 1 m where soil layers were recharged mainly by precipitation. Although Mongolian pine exhibits strong ecological adaptability, a trait that makes it ideally-suited for controlling desertification in sandy regions of northern China, water stress, crown dieback, or even tree mortality are likely to occur under conditions of extremely low precipitation.